1. Field of the Invention
The present invention relates generally to apparatus for generating electric current. More particularly, the invention concerns an integrated system comprising a fuel cell which is interconnected with a pair of turbocompressors in a manner so as to increase the pressure of atmospheric oxygen fed to a fuel cell by utilizing the potential energy of the exhaust of the fuel cell as well as the potential energy of a source of fuel under pressure.
2. Discussion of the Invention
A fuel cell is a device in which the energy released in the oxidation of a conventional fuel is made directly available in the form of an electric current. Although the principle of the fuel cell was formulated as long ago as 1894, it is only in recent years that reasonably efficient fuel cells have been constructed and put to practical use for generating electricity in many applications, including vehicular propulsion.
Typically the fuel cell operates by bringing together a fuel gas, as for example hydrogen, and oxygen, typically taken from the atmosphere. Generally the hydrogen fuel is bottled under high pressure, liquefied, or chemically bound. The reactants are combined in a vessel under some convenient combination of pressure and temperature, for example, 10 to 40 pounds per square inch gauge and 220 degrees Fahrenheit. The so called Proton Exchange Membrane (PEM) fuel cell represents a typical prior art fuel cell usable in the apparatus of the invention, but the invention is not limited to the use of this type of cell.
A typical PEM fuel cell operates by combining oxygen and hydrogen across a membrane while developing an electromotive potential between two electrodes placed on either side of the membrane. The process also releases heat which increases the temperature of the reacting fluids and tends to vaporize the newly formed water.
Although the oxygen could be supplied in pure molecular form (this is done for some applications, e.g. space and submarine), earthbound fuel cells use atmospheric air as the source of the oxygen, which is pumped into the fuel cell together with the nitrogen, carbon dioxide, argon, moisture and any other trace gases present in the air. The excess water formed in the cell reaction is generally removed as water vapor entrained in the exhaust of the non-reacting nitrogen, carbon dioxide, etc. from the fuel cell back to the atmosphere.
In practice the removal of the water in the form of saturated vapor requires a volume of entraining gas larger than the volume containing the stoichiometric quantity of oxygen reacted to form the water to be removed. Typically, complete removal of the water requires a volume of air approximately 1.5 times larger than stoichiometric. This imbalance has the consequence that the exhaust gas also contains a percentage of unreacted oxygen. Assuming that atmospheric air contains, in round numbers, 80% nitrogen and 20% oxygen (and neglecting for simplicity the minor constituents) the reaction: EQU 2H+O.fwdarw.H.sub.2 O+heat
or indicating complete molecules, EQU 2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O+heat
becomes EQU 2H+1.5O+6N+. . . .fwdarw.H.sub.2 O+0.50+6N+heat
or indicating complete molecules, EQU 4H.sub.2 +3O.sub.2 +12N.sub.2 +. . . .fwdarw.4H.sub.2 O+O.sub.2 +12N.sub.2 +. . .+heat
The enthalpy of the exhaust gas is available for use in a turbine, which can operate with a typical efficiency of 85-90% when built in accordance with the state of the art for the dimensions and RPM appropriate to the applications.
The power developed by the turbine is coupled to a centrifugal compressor matched to the turbine RPM through a shaft comprising a gas bearing as illustrated for example in FIG. 7 of U.S. Pat. No. 4,808,070, which issued to the present inventor. Since the efficiency of the state-of-the art centrifugal compressor is typically 70-75%, the power developed in the turbine may not be sufficient (depending on the temperature of the exhaust flow) to compress the air to the desired pressure. In any event, a single-stage centrifugal compressor has a practical limit of about 1.8:1 for the ratio of absolute pressure of the discharge over the inlet, or 12 psig. discharge for an atmospheric inlet. If the desired operating pressure of the fuel cell is higher than this, two or more stages will be required, and the upper stages can be driven by expanding hydrogen through turbines rather than through a regulator valve. For simplicity a two-stage system is discussed hereafter.
As a general rule, in operating a fuel cell of conventional construction, the pressure of the hydrogen must be lowered from that existing in the storage bottle, while the pressure of the oxygen taken from atmosphere must be increased above atmospheric pressure. In prior art devices, these pressure changes are accomplished by means of a regulator valve, in the case of the hydrogen, and by a compressor, in the case of the oxygen. Generally, in operating the prior art fuel cell, the power needed to run the compressor is taken from the electrical output of the fuel cell and can constitute a substantial fraction thereof. In addition, the compressor used in prior art applications requires lubrication which is typically supplied by various types of lubricating means. The lubricating means generally increases the complexity, weight and cost of the operating system and offers a finite possibility of undesirable contamination of the fuel cell with lubricants.
In carefully analyzing fuel cell operation it became apparent to the present inventor that there are at least two sources of energy that can be used to directly drive a compressor. One such source is the potential energy of the exhaust gas expanding from the fuel cell operating pressure to atmospheric pressure. Another source of energy is the potential energy of the compressed hydrogen expanding from the reservoir to the fuel cell itself. In typical prior art system, this latter source of energy is dissipated by dropping the pressure through one or more regulating valves. However, as will be better understood from the discussion which follows, this energy can be beneficially recovered by expanding the hydrogen through one or more turbines which are directly connected to one or more centrifugal compressors. It is this novel feature which comprises an important aspect of the apparatus of the present invention. More particularly, a significant contribution to the present invention was the realization by the present inventor that part of the pressurized hydrogen normally used as the fuel source effectively could be exploited to drive a turbine and the energy thus derived could be used to drive a centrifugal compressor which, in turn, could function to increase the pressure of the oxygen taken from atmosphere and supplied to the fuel cell.
Another important aspect of the present invention involves the use in the apparatus of the invention of a novel gas bearing turbine of the character described in U.S. Pat. No. 4,808,070 issued to the present inventor. Use of this highly novel, non-lubricated fluid bearing turbine not only provides the high operating efficiency required in the present application, but also elegantly solves the contamination problem encountered as a result of the use of conventional lubricated turbines.